In the vast majority of pcs sold these days, extended memoryis the type of memory over and above the first megabyte (which is the conventional memory). A PC must have at least an 80286 microprocessor(meaning that an 80386 or 80486 will work, too) to use extended memory. But even then, DOS doesn't recognize extended memory-to use it, your software has to incorporate a DOS extender.

Extended memory isn't the same thing as expanded memory—these two terms have specific meanings, and you have to know which one to ask for.